A first aspect of the invention relates to a compound of formula (1A) or (1B), or a pharmaceutically acceptable salt thereof, Wherein R1 is a group selected from alkyl, monocyclic heterocycloalkyl, bicyclic heterocycloalkyl and cycloalkyl, each of which is optionally substituted, and wherein X1, X2, X3 and X4 are as defined herein. Further aspects relate to pharmaceutical compositions, therapeutic uses and process for preparing compounds of formula (1A) and (1B).